Objective: The present study investigated the effects of the Er:YAG laser's different pulse repetition rates on temperature rise under various primary dentin thicknesses. Background data: The Er:YAG laser can be used for restorative approaches in clinics and is used to treat dental caries. There are some reports that explain the temperature rise effect of the Er:YAG laser. Recently, the Er:YAG laser has been found to play an important role in temperature rises during the application on dentin. Methods: Caries-free primary mandibular molars were prepared to obtain dentin discs with 0.5, 1, 1.5, and 2 mm thicknesses (n = 10). These discs were placed between the Teflon mold cylinders of a temperature test apparatus. We preferred three pulse repetition rates of 10, 15, and 20 Hz with an energy density of 12.7 J/cm(2) and a 230 mu s pulse duration. All dentin discs were irradiated for 30 sec by the Er:YAG laser. Temperature rises were recorded using an L-type thermocouple and universal data loggers/scanners (E-680, Elimko Co., Turkey). Data were analyzed by two-way ANOVA and Tukey tests. Results: Whereas the lowest temperature rise (0.44 +/- 0.09 degrees C) was measured from a 10 Hz pulse repetition rate at a dentin thickness of 2 mm, the highest temperature rise (3.86 +/- 0.43 degrees C) was measured from a 20 Hz pulse repetition rate at a 0.5 mm dentin thickness. Conclusions: Temperature rise did not reach critical value for pulpal injury in any primary dentin thicknesses irradiated by a high repetition rate of the Er:YAG laser.